Mixture solubilities are of particular importance when considering supercritical processes for separation of multicomponent mixtures and extraction of mixtures from solid matrixes, such as cleaning of spent adsorbents or remediation of contaminated soils. An investigation of the solubility limits of two priority pollutants, hexachlorobenzene and pentachlorophenol, was performed using a novel solubility apparatus. Mixture solubilities were found to decrease substantially in the case of hexachlorobenzene. A thermodynamic model employing the Wong-Sandler mixing rules was used for prediction of the solubility data. This model was shown to work better than the traditional van der Waals quadratic mixing rule for the binary systems and correlated well with measured pentachlorophenol partial molar volumes.